In a bipolar semiconductor device, as shown in FIG. 14, using a silicon nitride film formed by plasma CVD (hereinafter referred to as “a plasma nitride film”) as a final passivation film in order to suppress a deterioration of hFE of a transistor due to an external environment such as moisture and to obtain reliability is well-known, which is shown in “Reliability Technology of Semiconductor Device” published by JUSE Press, Ltd.
However, since a film stress of the plasma nitride film is high, a lifetime of wirings may be lowered. In addition, property variation of resistance of polysilicon due to residual hydrogen in the plasma nitride film while forming the film is reported recently in Jpn. J. Appl. Phys. Vol. 32 (1993) pp. 3734-3788 Part 1, No. 9A, September 1993.
As the conventional art to improve the property variation of resistance of polysilicon due to the residual hydrogen in the plasma nitride film, a prior art 1 shown in FIG. 15 is proposed, which is disclosed in Japanese Patent No. 2748070 corresponding to U.S. Pat. No. 5,470,764. Amount of hydrogen diffused into polysilicon resistor films 108a and 108b through oxide films 104 and 105, which are arranged under each of the films 108a and 108b, depends on thicknesses of the oxide films 104 and 105. Therefore, silicon nitride films 121, which are formed by low pressure CVD, are arranged as hydrogen ion shielding films under each of the polysilicon resistor films 108a and 108b. Thereby, diffusing of hydrogen from the base oxide film is suppressed and variation of resistance of the polysilicon resistor films is decreased.
Moreover, a prior art 2 shown in FIGS. 16A to 16C is proposed in Japanese Patent No. 2748070 as another art. According to the art, unlike the prior art 1, a polysilicon resistor element 207 formed over a semiconductor substrate, in which a bipolar device is formed, through a lower silicon nitride film 216, is covered by a covering oxide film 208. After that, a silicon nitride film 210 formed by low pressure CVD is formed to cover a surface of the semiconductor substrate including a junction separation layer. Thereby, diffusing of hydrogen into the polysilicon resistor element is suppressed and the variation of resistance of the polysilicon resistor element is decreased.
On the other hand, as an effect of the plasma nitride film with respect to hFE of a general bipolar transistor shown in FIG. 17, hFE of the bipolar transistor increases by the residual hydrogen in the plasma nitride film as shown in FIG. 18. In FIG. 18, XVIIIA represents hFE of the NPN transistor after forming the plasma nitride film, XVIIIB represents hFE of the NPN transistor after forming the aluminum wiring, XVIIIC represents hFE of the PNP transistor after forming the plasma nitride film, and XVIIID represents hFE of the PNP transistor after forming the aluminum wiring. In other words, increasing of hFE represents increasing of a gain of the bipolar transistor, and thereby, it is an advantage in terms of the transistor property.
However, a threshold value of a parasitic MOS type element (hereinafter referred to as “a field threshold voltage Vt”, which is shown in a3 of FIG. 17, including a p-type diffusion layer 6 as a collector of a PNP transistor, a p-type diffusion resistor layer 7, a laminated portion including a thick thermal oxide film 9 and a thick CVD oxide film 10 (hereinafter referred to as “a field portion”) and an aluminum wiring arranged over the laminated portion, decreases due to the residual hydrogen in the plasma nitride film, as shown in FIG. 19. The amount of decrease of the field threshold voltage Vt due to the residual hydrogen in the plasma nitride film depends on plane orientation of a substrate. In case that total film thickness of the oxide films in the field portion is 1.5 μm, when a silicon substrate having a (111)-plane orientation and an off-set angle in a range between 3 degrees and 4 degrees, which is used generally for a bipolar integrated semiconductor element, the field threshold voltage Vt decreases about 10V because of the residual hydrogen in the plasma nitride film.
In addition, in a capacitor in which an n-type diffusion layer 8 arranged in an n-type epitaxial layer 4 having a (111)-plane orientation and an off-set angle in a range between 3 degrees and 4 degrees is used as one electrode, as shown in FIG. 17, because of plane orientation and formation of an capacitor oxide film 11 on the n-type diffusion layer 8 having high impurity by thermal oxidation, it is difficult to obtain a reliable capacitor.
When the field threshold voltage Vt is low, it becomes easy for the parasitic element to operate. In other words, it means that malfunction of the semiconductor device becomes easy to occur. The substantial decreasing of the field threshold voltage Vt due to the residual hydrogen in the plasma nitride film causes serious reliability degradation for an in-vehicle high breakdown voltage bipolar semiconductor device having a withstand voltage of greater than or equal to 24 volts.
In order to increase the field threshold voltage Vt without decreasing the withstand voltage of the junction layer, making thicker a total thickness of the oxide films in the field portion is known generally. In case that the thickness of the oxide films in the field portion is further increased, wiring disconnection may be caused. In particular, in case that the thickness of the thick CVD oxide film 10 is further increased, cracks may generate in the CVD oxide film 10 and reliability degradation of the semiconductor device due to the cracks may occur.
On the other hand, with respect to suppressing the influence of the residual hydrogen in the plasma nitride film to elements of the bipolar transistor or the like, the prior art 2, which discloses that the surface of the semiconductor substrate is covered by the silicon nitride film formed by low pressure CVD, may be excellent. However, according to the prior art 2, the aluminum wiring is covered by the silicon nitride film formed by low pressure CVD, which has a large film stress, and the plasma nitride film, which is formed for protecting the elements. Therefore, a stress migration may generate and the lifetime of the wiring may be lowered significantly. In addition, in case that a thin film resistor such as metal and metal silicide is formed over the semiconductor substrate, a property of the thin film resistor may be changed because of the film stress of the silicon nitride film.
Moreover, in the bipolar semiconductor device, as an art to reduce the total film thickness of the field portion and improve a defect of the field portion due to gamma-rays, the prior art 3 shown in FIG. 20 is proposed, which is disclosed in Japanese Patent No. 2545953. In this art, a silicon nitride film 306 is formed between a thermal oxide film 305 and a CVD oxide film 307, and a p+ region layer 304 is formed in a base region of an NPN transistor. Thereby, an inversion at a surface of the p-type region due to gamma-rays is suppressed. According to the structure of the prior art 3, the total film thickness of the field portion can be decreased, and an aluminum wiring over the silicon nitride film 306 is formed with the CVD oxide film 307 interposed therebetween. Therefore, reliability of the wiring can not be deteriorated. However, since the p+ region layer 304 is formed in the base region, the withstand voltage may be lowered when this art is applied to an in-vehicle high breakdown voltage bipolar semiconductor device.
Moreover, in the above-mentioned prior art 2 and the prior art 3, since the surface of the semiconductor substrate is covered by the silicon nitride film having the high film stress, the element property variation due to the film stress generates. Particularly, when the prior arts 2 and 3 are applied to the bipolar semiconductor device shown in FIG. 14, malfunction may occur because hFE of a parasitic NPN transistor, which includes a junction separation layer including an embedded p-type layer 2 and a p-type separation diffusion layer 5 as a base, is increased, and the reliability of the bipolar semiconductor device is significantly decreased.
In the above-mentioned prior arts, the decrease of the field threshold voltage Vt due to the residual hydrogen in the plasma nitride film and the decrease of the effect to the bipolar transistor including the parasitic element can not be compatible. In addition, the decreasing of the reliability of the aluminum wiring and variation of the resistance value of the thin film resistor such as the metal or the metal silicide can not be suppressed.